Showerhead supporting arm



FIG. 1 is a front end, left side and bottom perspective view of a showerhead supporting arm showing my new design;

FIG. 2 is a left side elevational view thereof;

FIG. 3 is a right side elevational view thereof;

FIG. 4 is a front end elevational view thereof;

FIG. 5 is a rear end elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof. 

The ornamental design for a showerhead supporting arm, as shown and described. 